High quality Ib gem diamond single crystals were synthesized in cubic anvil high-pressure apparatus (SPD6 × 1200) under 5.4GPa and 1230℃-1280℃. The (100) face of seed crystal was used as growth face, and Ni70Mn25Co5 alloy was used as solvent/catalyst. The dependence of crystal quality and β value (the ratio of height to diameter of diamond crystal) on synthesis temperature was studied. When the synthesis temperature is between 1230℃ and 1280℃, the β value of the synthetic high-quality gem diamond crystals is between 0.4and 0.6. The results show that when the β value is between 0.4 and 0.45, the synthetic diamonds are sheet-shape crystals; however, when the β value is between 0.45 and 0.6, the synthetic diamonds are tower-shape crystals. In addition, when the β value is less than 0.4, skeleton crystals will appear. When the β value is more than 0.6,most of the synthetic diamond crystals are inferior crystals.Supported by the National Natural Science Foundation of China under Grant Nos 50572032 and 50731006.